1. Field of the Invention
Disclosed herein are zinc sulfide (ZnS) single crystals and multi-grain ZnS crystals that possess physical properties effective for many optical applications. More specifically, the single crystals and multi-grain crystals are of pure or a substantially pure wurtzite structure with a high chemical purity and a high crystalline perfection and possess physical properties superior to commercially existing ZnS crystal materials. The crystals can be used to fabricate components or devices including, but not limited to, optical components (in infrared (IR) & visible spectrum, in the wavelength range of 0.34-14 μm), photoluminescence devices, cathode luminescence devices, electroluminescence devices, semiconductor devices, and laser gain medium for IR lasers (1-5 μm in wavelength). Also disclosed are methods of producing the aforementioned ZnS single crystals or multi-grain ZnS crystals.
2. Description of the Related Art
ZnS is a versatile material that has a variety of applications. Representative of current applications for ZnS are:
(1) High purity ZnS crystals are transparent in the visible and IR spectrum ranges (approximately 0.34-14 μm in wavelength) and can be used to fabricate optical components, such as forward looking infrared (FLIR) windows and other optical components operating in that wavelength range.
(2) ZnS doped with one or more impurities is a photoluminescence material useful as a scintillator or a phosphor for detecting or otherwise imaging (or image intensifying) UV lights, X-rays, gamma-rays and neutrons. Currently, only powder-based or thin-film-based ZnS polycrystalline materials are used for this application.
(3) ZnS doped with one or more impurities is an important cathode-luminescence material that is widely used in phosphor screens in display devices such as cathode ray tubes (CRTs), CRT-type TV sets, field-emission displays, and image intensifiers. Currently, only powder-based or thin-film-based ZnS polycrystalline materials are used for this application.
(4) ZnS doped with one or more impurities is also an important electroluminescence material that is used to fabricate a variety of electroluminescence devices, such as light emitting devices and flat panel displays (including single-color, multi-color, or full-color displays). Presently only thin-film-based polycrystalline ZnS is used for this application.
(5) As a II-VI wide band gap semiconductor material, ZnS, in a form of a single crystal or a multi-grain crystal (with an average crystal grain size larger than 1 mm), by itself or in combination with other II-VI, III-V, IV and other semiconductor materials, can be used to fabricate a variety semiconductor devices, such as light emitting diodes (LEDs), laser diodes (LDs), high frequency devices, high power devices, UV detectors, solar cells and photoconductive devices.
(6) A ZnS single crystal or a multi-grain ZnS crystal can also be used as a substrate (or template) for epitaxial growth of thin films (<10 μm in thickness) or thick films (10-1000 μm in thickness) of group III-nitride semiconductors, such as GaN, AlN, InN, and their alloys, and II-VI semiconductors, such as ZnO, ZnS, ZnSe, or as seeds (or templates) for growth of bulk crystals (larger than 1 mm in thickness in the growth direction) of group III-nitride semiconductors, such as GaN, AlN, InN, and their alloys, and II-VI semiconductors, such as ZnO, ZnS, ZnSe, that can be used to fabricate semiconductor devices including blue, UV and white LEDs, blue and UV LDs, UV detectors, high frequency devices, solar cells and photoconductive devices.
(7) A ZnS single crystal of high crystalline quality doped with a transition metal ion dopant (e.g. Co+2, Cr+2, Fe+2) can be used as a laser gain medium for diode-pumped lasers operated in the near IR region (approximately 1-5 μm in wavelength).
Currently, powder-based or thin-film-based ZnS polycrystalline materials are usually used for these applications. A ZnS single crystal or a multi-grain ZnS crystal of pure or substantially pure wurtzite structure, which is not available commercially to date, may have a better performance.
ZnS typically crystallizes in one of two crystal structures: cubic (sphalerite, or zinc blende) and hexagonal (wurtzite). The currently commercially available ZnS materials are polycrystalline ZnS produced via a chemical vapor deposition (CVD) method, and CVD ZnS processed via a hot isostatic pressing (HIP) process (such as Cleartran ZnS, Multi-spectral ZnS) of mainly the cubic structure with crystal grain sizes in the range of approximately 3-10 μm for the CVD-ZnS and approximately 100-200 μm for Cleartran ZnS or multi-spectral ZnS materials. (Cleartran is a trademark of Morton International, Inc. Chicago, Ill.).
In addition to the cubic and hexagonal crystal structures, ZnS has many polytypes. Examples of ZnS polytypes are 4H, 6H, 8H, etc., (the hexagonal polytypes), and 12R, 24R, 30R, etc., (the rhombohedral polytypes). The cubic ZnS (also called 3C polytype) and the wurtzite-structure ZnS (also called 2H polytype) crystals are important for practical use. ZnS of different polytypes have different physical properties. For example, the band gap energy of cubic ZnS is about 2.7 eV, and the band gap of wurtzite-structure ZnS is about 2.8 eV. A ZnS crystal containing more than one polytype, i.e. mixed polytypes, may degrade its physical properties. Therefore, for many applications, it is favorable to use a ZnS single crystal of a pure or substantially pure polytype.
A first method to grow ZnS single crystals is melt-growth. ZnS melts congruently at about 1830° C. and can be grown from its melt. Melt-grown ZnS crystals were found to be of wurtzite structure. However, because the vapor pressure of ZnS at its melting point is very high (approximately 3.7 atm), ZnS crystal growth from the melt using a melt growth method, such as a Bridgman technique, has to be carried out at a very high Ar gas pressure (˜50 atm) and the growth process is a safety hazard and difficult to control. Further, due to contaminants, mainly from the crucible material contacting the ZnS melt, melt-grown ZnS crystals were found to contain excessive amounts of undesirable impurities and crystalline defects.
A second growth method is a sublimation physical vapor transport (PVT). A PVT growth is essentially a sublimation and re-condensation process. A source material placed at the high temperature end of a crucible sublimes and the vapor species travel and re-condense onto the other end of the crucible at a lower temperature to form a crystal. Because ZnS sublimes at high temperatures (>1100° C.), ZnS single crystals can be grown via a sublimation PVT technique at a temperature significantly lower than the melting point of ZnS. Because ZnS has an appreciable vapor pressure only at a temperature higher than 1200° C., growth of ZnS single crystals using a PVT technique in the prior art was carried out in a temperature range of 1200-1600° C. During the 1950s and the 1960s, large single crystal grains up to 15 grams in weight were harvested from large multi-grain crystal boules grown at high growth temperatures of about 1400-1550° C. range and the ZnS crystals had a wurtzite structure. But the wurtzite-structure ZnS single crystals produced in this way were impure and of low crystalline perfection. For example, chemical etching revealed that etch-pit densities were in range of 105-108 cm−2. The impurities and defects are believed due to impure ZnS source materials and contaminants from the growth crucibles (containers) made of a mullite tube or a fused silica tube.
More recently, growth of ZnS single crystals of cubic structure has been attempted via a low-temperature (˜1200-1250° C.) PVT growth technique. As a result, ZnS single crystals, up to 30 mm in diameter, of predominantly cubic structure with a small amount (less than 5% by volume) of wurtzite-structure can be produced. At present, predominant cubic-structure ZnS single crystals of about 10×10 mm2 in area are commercially available in small, research quantities, such as from RMT Ltd. of Moscow, Russia. But, the growth rate at these low temperatures is quite low, typically less than 0.1 mm/hr.
Another method to grow ZnS single crystals of cubic structure is a chemical vapor transport using I (iodine) as a transport agent at a temperature of about 800-900° C. The cubic ZnS crystals produced in this way are doped by I (iodine) and therefore have an n-type conduction. Chemical etching revealed that etch-pit densities were in the range of 103-105 cm−2. Semiconductor devices built on such n-type cubic ZnS single crystals are disclosed in U.S. Pat. No. 5,274,248, Yokogawa, et al. However, the growth rate in iodine transport technique for ZnS is extremely slow (less than 0.05 mm/hr), which makes this method unfavorable for commercial volume production of ZnS single crystals.
The inventor believes that at present, there are no ZnS single crystals or multi-grain crystals made by any of the aforementioned methods that meet the requirements for many applications, such as optical components (in IR & visible spectrum in the wavelength range of 0.34-14 μm), photoluminescence devices, cathode luminescence devices, electroluminescence devices, and semiconductor devices.
In searching ZnS materials for forward looking infrared (FLIR) optical components operated in the wavelength range of 1-14 μm, researchers, mainly from Raytheon (Lexington, Mass.), produced ZnS polycrystalline materials for IR optical components via a CVD process, in which a Zn vapor and an H2S gas react to form ZnS and then deposit ZnS onto graphite substrates at a temperature of about 630-800° C. The as-grown CVD-ZnS, of essentially a cubic structure polycrystalline ZnS, had a poor transmission in the IR spectrum of 1-10 μm in wavelength, and had to be further processed via HIP to achieve an acceptable IR transmission for FLIR applications. The ZnS materials produced via CVD-HIP process are polycrystalline materials of cubic structure with an average grain size of 100-200 μm. At present, such ZnS materials are being marketed as Cleartran ZnS and Multi-spectrum ZnS. Cleartran ZnS and Multi-spectral ZnS materials still have some level of absorption and scattering likely due to residual structural defects and impurity clusters. Cleartran ZnS and Multi-spectral ZnS have a large bulk absorption coefficient (approximately 0.1-0.3 cm−1) at 10.6 μm, the wavelength at which a high power CO2 laser is operated, while a CVD zinc selenide (ZnSe) with a low bulk absorption coefficient (less than 0.001 cm−1) at 10.6 μm is currently the choice of materials for fabricating optical components for high power CO2 lasers. However, because of the high mechanical strength of ZnS, a ZnS with a low bulk absorption coefficient (less than 0.001 cm−1) is more favorable for fabricating optical components for high power CO2 lasers.
On the other hand, the CVD process for producing ZnS has a slow deposition rate (usually in the range of 0.05-0.1 mm/hr). CVD-ZnS crystals or Cleartran ZnS crystals are polycrystalline materials that cannot be used for applications such as semiconductor devices, laser mediums, substrate materials, etc. In addition, both the CVD process and the HIP process are a safety hazard and a costly process.
For optical applications, particularly IR applications including optics for FLIR and high power CO2 lasers, there exists a need of ZnS single crystals or multi-grain ZnS crystals with a better performance and a less cost to produce. There also exists a need for ZnS single crystals or multi-grain ZnS crystals of pure or substantially pure hexagonal structure for many non-optical applications. There is also a potential use of ZnS crystals of substantially pure wurtzite structure in many applications that have not yet been identified.